A ball valve is a kind of ball valves using a ball as the on-off member. The valve is fully open when turned to where the central through bore of the ball is coaxial with the valve passage, and fully closed when perpendicular to the valve passage. The turning of the on-off ball in its seats mounted by the valve body is controlled by a stem extending out of the valve body. The stem needs sealing to prevent medium from leaking along it. Some valves only have one seal either at stem cylinders or at stem shoulders, but most ball valves need the two.
The primary task of the stem shoulder of ball valves is to prevent the stem from being ejected out of the pressure boundary by internal pressure, and hence the stem shoulder seal and the stem cylinder seal are located respectively at the inside and the outside of a stem thrust step in valve bodies and fixed by a thread engagement of the nut and the threaded stem. The sealing of stem shoulders needs to provide an axial sealing stress on their shoulder, and the sealing of stem cylinders needs to provide a radial sealing stress on their cylinder. In other words, the two sealing stresses are perpendicular to each other and both originated by one thread engagement, so that it is difficult for them to be equally provided at the same time by one wrenching operation; i.e. unless specially designed, the stem shoulder seal and the stem cylinder seal can not get well seated at the same time during assembly to be two independent leak-free stem seals under ultimate medium pressure. Besides, the medium pressure on the stem end is an increasing load to the stem shoulder seal, and a decreasing load to the stem cylinder seal, so that the two seals, unless specially designed, can not also bear an identical ultimate medium pressure independently in services.
The conventional stem cylinder seal is a design for axially compressing the packing material around the stem in the stuffing box. Since the sealing materials are non-rigid and compressible, it is beyond any doubt that any section of shaped packing rings in sets will deform into a heap or lump of packing to lose their mechanical characteristic in rigid sections and only to be equivalent to a plain gasket when compressed to some extent. Since the Poisson's ratio (radical strain/axial strain) of usual sealing materials is less than 0.5, for example, PTFE has a Poisson's ratio of 0.46, any shape of packing rings compressed into a heap of packing and equivalent to a plain gasket can not effectively provide a sufficient radial sealing stress needed for the sealing of stem cylinders. It is imaginable that the flexible graphite in the stuffing box must deform into a lump of packing when compressed to some extent, and if such a lump could provide an radial sealing stress effectively, any premanufactured flexible graphite plain gasket could be never used as a seal of any end faces; however, the flexible graphite plain gasket has been widely used as flange seals, which means that the lump of flexible graphite can hardly radially deform or effectively provide a radial stress when axially compressed. Therefore, the conventional stem cylinder seal is only of an inefficient sealing design.
The conventional stem shoulder seal is a design of plain gaskets compressed axially. The stem shoulder sealing stress is generated from a thin plain gasket and proportional to its Poisson's axial strain, thereby increasing quickly in assembling; while the stem cylinder sealing stress is generated from a thick packing and proportional to its Poisson's radial strain less than its Poisson's axial strain at least by 50%, thereby increasing slowly in assembling; i.e. in assembling, the stem shoulder seal and the stem cylinder seal can not get well seated at the same time by one compression force from the thread engagement of the nut and the threaded stem. In services, the stem shoulder sealing is strengthened and the stem cylinder sealing is weakened under medium pressure; i.e. in service, the stem shoulder seal and the stem cylinder seal can not also function effectively at the same time. As a result, very often the stem cylinder seal can not yet function when the stem shoulder seal material is compressed to be crushed.
The previous patent CN1920363A attempts to use a triangular section of stem cylinder seals and a ball wedge/spherical seat mating of stem shoulder seals to ensure that the two seals get well seated automatically at the same time during assembly to bear an identical ultimate medium pressure independently in service. However, the ball wedge/spherical seat mating causes the stem shoulder seal to have a slightly big operation torque.